In order that the flow of outside air may be able to be used to convey air to and from the space it is necessary for the inflowing air to be caught by an opening which is facing in the direction of the wind, while the air which is to be conveyed out of the space is permitted to flow out at the leeward side. Ventilators, which use this principle as a result of the fact that a hood is permitted to swing by means of a weather vane, were known previously. With such a construction, the effect is achieved that the hood always has an intake opening in the direction of the wind and an outflow opening at the leeward side. Nevertheless, the operation of such ventilators depends entirely on the hood being free to swing and ventilators of this type have not proved reliable, particularly when there is a risk of the formation of ice and driving snow. Other attempts at providing passages in a fixed housing, which provide inwardly and outwardly directed flows have not led to the intended effect and provide too weak a ventilation for the majority of purposes as well as being dependent on the wind direction in an unintended manner.